As technology advances, the volume of data around the world is growing at an exponential rate. Everything from increased usage of the global information networks (e.g. the Internet), video conferences, and mobile phones relies on efficient data transfer. Reductions in the distance that electrons need to travel within and between components have provided the dramatic increases in device speeds. Increases in the speed of electronic devices through the 1980s and 1990s largely resulted from reductions in size of microelectronic components. However, microelectronics communications networks have physical limitations that effectively limit the volume of data that can be transferred. As devices encroach upon the physical limits of component density and complexity, device reliability and speed advances for new devices are declining.
Optical communication of data (e.g., sending photons through optical fiber, rather than sending electrons through wire) is already widely implemented for certain connections and communications. Optical connections, optical switching, and all-optical circuits provide ways for photons to travel and be routed, in place of electrons traveling through electronic circuits. Wavelength division multiplexing (WDM) provides a way to send even more data through optical components (such as fiber) by mixing light of different wavelengths in the same fiber. A demultiplexer separates a specific wavelength of light from a fiber. Time division multiplexing (TDM) provides another way to send data through optical components. In TDM, bits associated with different channels are interleaved in the time domain to form a composite bit stream. A TDM demultiplexer separates the channels by providing a set of gates which allow the interleaved portions of the optical signal representing the channels to be coupled into separate, single channel fibers in response to a clock signal. TDM demultiplexing requires a clock signal and the ability to provide specific time delays to the clock signal.
Photonic devices and microphotonics provide significant potential for furthering the advancement of technology devices historically served by microelectronics because they also enable large volumes of data to travel along optical fibers and be routed to their final destinations. A primary reason that all-optical circuits have not yet been implemented is that there are manufacturing problems related to photonic device fabrication, such as meeting index of refraction specifications. The small feature size required for photonic devices, as well as small tolerances for physical specifications of photonic devices, have delayed the discovery and use of mass manufacturing techniques for these devices.
Photonic crystals are structures that restrict the propagation of particular wavelengths by the use of destructive interference and can be designed for very complex routing of light. While optical fiber cannot effectively route light through sharp 90-degree turns, or through complex three-dimensional devices, photonic crystals can be used for these purposes. Photonic devices in general and photonic crystals in particular are applicable to lasers, filters and light-emitting diodes and are also applicable in WDM (wavelength division multiplexing) applications. In WDM, light of many different wavelengths travels along an optical fiber and the different wavelengths of light have different destinations.
A photonic crystal may be used for a specific application which benefits from design control of the bandgap, those wavelengths that are “forbidden” (do not pass through the structure) and/or the narrow band of transmitted wavelengths within the “forbidden” range of wavelengths.
Photonic crystals may be 1D (one dimension), 2D (two dimension) or 3D (three dimension) periodic structures. Such periodic structures may comprise periodic holes, periodic posts, or the periodic occurrence of a particular index of refraction. Periodicity distinguishes periodic photonic crystals from other photonic devices. When the product of the lattice constant (pitch) and the index of refraction is on the order of the wavelength of the light traveling in the structures, photonic crystals usually demonstrate some unique characteristics. For example, some wavelengths are strongly enhanced and some wavelengths are strongly suppressed. These characteristics can be used to manipulate light.
One kind of photonic crystal is a photonic bandgap crystal in which light at specific “forbidden” wavelengths (those wavelengths within the photonic bandgap) are rejected by the photonic crystal. The introduction of a defect into a photonic bandgap crystal creates a narrow transmission peak where a specific, narrow band of wavelengths of light within the bandgap range is allowed to propagate while all other wavelengths within the photonic bandgap are forbidden. As the light travels through the photonic bandgap crystal, the light in the desired band of wavelengths is transmitted by the photonic crystal and routed to its destination. The range of wavelengths that do not pass through the photonic bandgap crystal may be relatively broad, for example 1300 μm-1700 μm, while the band of passed wavelengths may be relatively narrow, for example, 1625 μm-1650 μm.
Photonic devices are fabricated using the known methods of x-ray or optical lithography. X-ray and optical lithography allow manufacturers of those devices to create very small feature sizes. Currently, photonic crystals are commonly manufactured using GaAs and GaAlAs or layered Si and SiO2. Alternatively, any solid-state material may be used as a photonic crystal as long as it has low absorption of the wavelength traveling in it.